Turning the Moon Into a Particle Detector

While astronomers clamor for bigger and bigger telescopes, physicists have quietly brought into play the largest instrument yet: the moon. Of course, such an enormous (not to mention opaque) chunk of rock isn't much use for gathering light. But a 40-year-old theory has shown a way to turn it into a neutrino detector, and a new study shows that such a project is feasible.

Neutrinos are haughty particles, so reluctant to interact with matter that they often pass right through Earth unhindered--and that makes them hard to detect. In 1961, Soviet physicist Gurgen Askaryan suggested that if an incoming neutrino is energetic enough, it will generate a pattern of coherent, polarized radio and microwave emissions when it happens to interact with a chunk of mass.

David Saltzberg, a physicist at the University of California, Los Angeles (UCLA), and his colleagues realized that this "Askaryan effect" might give them a unique opportunity to detect ultrahigh-energy neutrinos coming from outside our galaxy. If a superfast neutrino passes through most of the moon and then strikes an atom near its surface, the collision would generate telltale radio waves that Earth-based antennas could detect. Astrophysicists believe that such high-velocity neutrinos exist but have never identified them.

The only problem was that the Askaryan effect had never been tested in the lab in a solid medium like the moon's surface. So Saltzberg's team shot a powerful beam of gamma rays, a convenient neutrino substitute, at the Stanford Linear Accelerator into a 3.5-ton box of sand like that on the lunar surface. The beam whipped up particle showers which created radio waves that matched Askaryan's predictions.

"This [effect] has been talked about for more than 30 years," says team member Dawn Williams, a physicist at UCLA who described the experiment at the American Physical Society meeting in Washington, D.C. on 28 April. David Besson, a physicist at the University of Kansas, Lawrence, hails the test as "the first demonstration of the Askaryan effect in a dense medium."

Encouraged by their success, Saltzberg's team turned its attention skyward. Borrowing downtime on radio antennas in the Mojave Desert that NASA ordinarily uses to communicate with spacecraft, the scientists swiveled the dishes toward the moon to listen for the radio waves from high-energy neutrino strikes. In 30 hours of observations, they detected no sure signals. But it's still early in the game, Besson says; with 120 hours still to go, he expects that something will turn up soon.